1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent images and a process for producing the toner. More particularly, it relates to a toner for developing electrostatic images which comprises a toner particle produced by polymerizing a polymerizable monomer composition containing at least a polymerizable monomer, and a process for producing the toner comprising polymerizing a polymerizable monomer composition containing at least a polymerizable monomer, and obtaining the toner particle.
2. Related Background Art
There are many known techniques for electrophotography, as described, for example, in the specification of U.S. Pat. No. 2,297,691. In general, it is a process in which an electric latent image is formed on a photosensitive body utilizing a photoconductive material by various means, the latent image is developed by the use of a toner, and if necessary, the toner image is transferred onto a recording material such as paper, then it is fixed by heat, pressure or a solvent vapor and the like, to produce a copy. Various methods have been proposed so far, for developing the latent image by using the toner and for fixing the toner image, and each image formation process employs an appropriate method.
Recently, the electrophotographer has been required to provide a copy of higher image quality at a high speed.
Generally, a process referred to as pulverization has been known for producing a toner, in which a coloring material such as a dye and a pigment, and an additive such as a charge controlling agent are added to a thermoplastic resin, and melt-mixed to provide a homogeneous dispersion. The dispersion is pulverized and classified using a pulverizer and a classifier to produce a toner having a desired particle size.
With the toner produced by the pulverization process, there is a restriction in addition of a releasing agent such as a wax. That means, in order to assure the dispersibility of the releasing agent to be at a sufficient level, 1) a certain level of viscosity should be maintained at the kneading temperature with the resin, and 2) the content of the releasing material should be less than about 5 parts by weight, and so on. Due to these restrictions, the fixing capability of the toner produced by the pulverization process is limited.
On the contrary, a toner produced by suspension polymerization, i.e. a toner produced by polymerization method (hereinafter referred to as a polymerized toner) has no such limitation, and can encapsulate the wax and provides good fixing capability and offset resistance.
However, so far, the fixing capability of the polymerized toner has not been examined sufficiently from the perspective of high speed, high image quality, and full color copying and thus the features of the polymerized toner have not yet been fully drawn.
Particularly, since the polymerized toner of the conventional structure sometimes shows inferior blocking resistance when it contains a large amount of a wax, it is required to be improved. For example, in the specification of Japanese Patent Laid-Open Application No. 2-273758, acceleration of pseudo-capsulation is carried out. However, it has been disclosed that such pseudo-capsulation also requires further improvements when the toner contains a large amount of a wax.
In addition, for such encapsulation of a wax having a low melting point, it is required to increase the amount of the wax in order to improve the fixing capability. In such a case, it is difficult to encapsulate the wax completely, and even though the blocking characteristics can be retained at a certain level, the fluidity of the toner becomes insufficient.
As for the fluidity and the coagulation of the toner, a reactive polyester containing polymerized toner is proposed in the Japanese Patent Laid-Open Application No. 56-116042, and in the Japanese Patent Laid-Open Application No. 56-116043, and a saturated polyester containing polymerized toner is proposed in Japanese Patent Laid-Open Application No. 60-238846.
The toners produced by these methods have solved the problem of the fluidity to some extent. However, when a large amount of a wax is contained, the fluidity of the toner is still insufficient and the blocking resistance or granulation capability are lowered.
It is thought that the granulating system becomes unstable due to the inferior compatibility of the polyester resin with the wax and thus the content of the wax and that of the polyester in each toner particle fluctuate.
In order to decrease the particle size of the toner particle to improve the image quality, the polymerization process is more advantageous than the pulverization process from the view point of the toner yield and the energy required for the production. However, it was determined that polymerized toners are difficult to be sufficiently charged for providing high image quality under a stabilized condition. As the polymerized toner is granulated in an aqueous medium, the polar substance such as the charge controlling agent is localized on the surface. Due to this phenomenon, even when a charge controlling agent is added in a small amount, the toner particles are sometimes excessively charged. When the amount of the charge controlling agent is further decreased, it leads to a problem of delayed charge build-up. Also, when a charge controlling agent of weak charging capability is used, the build-up of the charge tends to become a problem.